The present invention relates to equipment capable of measuring linear, angular, or combined linear and angular displacement. In particular, it relates to such devices which are operated on a digital basis and wherein the magnitude of such displacement is directly displayed or recorded in a prearranged digital code through suitable read-out and output means. The need for such systems, and in particular for such systems wherein the read-out is carried out without possibility of ambiguity and in a reliable manner, is very wide. A particular field of application to be discussed herein is the field of aerial and photogrammetry and of the taking of forest inventories. The system is, of course, in no respect limited to these applications.
As regards the taking of forest inventories, it has been found that the efficiency of such inventories can be increased by minimizing the tedious and expensive ground work required to correlate photo and ground estimates. Current research, therefore, is aimed at developing methods which will increase the reliability of photogrammetric measurements and estimates of tree characteristics. One technique which is becoming established is forest sampling by air photography from low altitudes (typically at scales between 1:1000 and 1:3000), which provides sufficient detail for accurate photo measurements of tree height and tree-crown dimensions which permit the estimation of tree diameter, volume and other statistics. But the conversion of these measurements to actual values requires exact knowledge of the photographic scale. Conventional methods based on ground controls are inefficient.
A radar altimeter has been developed which is capable of measuring distance between aircraft and the ground with a precision of .+-.1 percent, regardless of intervening vegetation. This continuous measurement is recorded on each photographic frame through the secondary optics of the camera. From the aircraft-to-ground distance and the focal length of the camera, the true dimensional relationship between the object and its image can be computed, provided the camera was vertical at the time of the exposure. Unfortunately, this is seldom the case and serious errors can be introduced by tilt. For example, when using small format (70mm) cameras and lenses of long focal length, a longitudinal tilt difference of only one degree between successive frames might cause a base deformation and height error of more than 20 percent. To overcome this difficulty, an airborne gyrostabilizer system can be used to continuously measure and record aircraft pitch and roll angles and the corresponding angles on the aerial camera which is rigidly mounted to the airframe. The longitudinal and lateral tilt angles measured at the gimbals of the gyroscope are recorded on each photographic frame through the secondary optics of the camera. Thus, the angles at which each exposure was made are immediately known. With the system of the present invention wherein a lack of ambiguity in the read-out is accomplished even at very high resolution, the required degree of accuracy of the tree measurements mentioned above can readily be achieved.
The present invention can, of course, find application also in conjunction with airborne gyrostabilizer systems wherein it is merely required to furnish an indication of the longitudinal and lateral tilt angles to the pilot. In this case, these angles are not recorded by a camera, but are displayed directly for the pilot's information.